Low-power wireless sensing nodes are key-components in the internet-of-things. Generally, these are formed by a group of passive sensors whose captured information is only transmitted when requested by other interrogating nodes in the network. One of the main sources of energy loss in conventional wireless sensing nodes is the power dissipated by their always active radios. The use of wake-up receivers has been proposed as a way to significantly reduce power dissipation, thus increasing the duration of their batteries.
Wake-up receivers are triggered by the presence of an RF (radio frequency) signal received by the antenna from the interrogating node. When the correct signal is received, the radio is turned on and the information captured by one or more sensors associated to the node is transmitted. The most important component of a wake-up receiver is the RF sensor that produces the triggering signal that turns on the radio when needed. Traditional RF sensors rely on diode-based rectifiers. These devices generate, from a portion of the received RF-power, a DC voltage controlling the operational status of the radio. However, because of their limited conversion efficiency and significant threshold voltages, these sensors require a significant input power to generate the DC voltage, thus demanding greater power.
Radio wakeup circuits have been designed in both passive (zero-power) and active low power mechanizations. Active low power approaches are commercially available [1] and continue to be optimized for sensitivity versus power consumption [2]. However, these traditional architectures cannot approach nW levels required for certain applications, such as the internet of things. A completely passive approach was proposed and analyzed by Gu and Stankovic [3] and in parallel by Brocato [4], who described it as a modern version of the AM crystal radio. The experimental work in [4] concluded that high Q resonators and significantly improved detectors are needed in order to achieve high sensitivity. More recent work by Parks et al. [5,6] develops wake-up circuits towards energy harvesting applications, but sensitivity is limited to the 10-20 μW level.
A switch has been developed [7] that employs a piezoelectric bimorph excited by vibration. The transducer is fed into the gate of an N-MOSFET. While the power with zero input is less than 10 nW, much more power is consumed for less than threshold inputs. Similarly, pressure and flow [8] activated piezoelectric switches were activated with FET power consumption stated at 1.5 μW. A light dependent resistor (LDR) has been proposed to wake up a magnetic sensor to determine the presence of automobiles [9]. However, continuous power consumption in the LDR was much greater than nW levels. Resonant switches for MHz applications with fast switching times have been developed [10-12].
There remains a need for radio wakeup circuits and receiver devices that operate at low power in the nW range.